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A-level chemistry my notes > Mechanisms > Flashcards

Mechanisms Flashcards

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1
Q

What condition is required for free radical substitution?

A

UV light

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2
Q

What is a free radical?

A

A reactive species with an unpaired electron

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3
Q

What happens during free radical substitution?

A

In the presence of UV light a halogen substitutes for a hydrogen atom in an alkane/halogenoalkane to produce a halogenolakane and a hydrogen halide.

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4
Q

What is heterolytic fission?

A

An endothermic process. Energy is added and the covalent bond breaks unevenly, both electrons going to the same atom

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5
Q

What is homolytic fission?

A

An endothermic process. Energy is added and the covalent bond breaks evenly, one electron going to each atom producing free radicals.

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6
Q

What are the steps in free radical substitution?

A

Initiation Propagation Termination

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7
Q

Describe initiation in free radical substitution

A

A halogen molecule splits homolytically in the presence of UV light. Molecule produces a free radical.

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8
Q

Describe propagation in free radical substitution

A

Molecule and a free radical react to produce a molecule and a free radical.

A) halogen free radical removes hydrogen from alkane.

B)Alkane free radical reacts with halogen

This is a chain reaction process.

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9
Q

Describe termination in free radical substitution.

A

Free radicals react to form a molecule.

End of the reaction

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10
Q

How can an excess of reactants be used to influence the products of free radical substitution?

A

Excess of alkane = one substitution

Excess of halogen = many substitutions.

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11
Q

What is a nucleophile?

A

A lone pair donator.

Generally a negative ion or molecule with delta negative charge.

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12
Q

What happens during nucleophilic substitution?

A

A nucleophlie replaces a halogen on a molecule.

Nucleophile is attracted to delta positive carbon caused by polar bond with an electronegative halogen.

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13
Q

Give the possible nucleophiles in nucleophilic substitution.

A

:OH- hydroxide ion

:CN- cyanide ion

:NH3 ammonia

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14
Q

Give the conditions for a hydroxide ion to act as a nucleophile in a nucleophilic substitution.

A

aqueous, warm, dilute

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15
Q

Give the conditions for a cyanide ion to act as a nucleophile in nucleophilic substitution.

A

ethanolic, warm

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16
Q

Give the conditions for ammonia to act as a nucleophile in nucleophilic substitution.

A

Excess concentrated ammonia dissolved in ethanol

(in a seeled container at constant pressure)

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17
Q

Name the organic product for nucleophilic substitution with an hydroxide ion

A

Alcohol

eg methanol CH3OH

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18
Q

Name the organic product for nucleophilic substitution with cyanide ions

A

Nitrile

e.g ethannitrile CH3CN

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19
Q

Name the organic product of nucleophilic substitution with a ammonia nucleophile

A

amine

e.g ethylamine CH3CH2NH2

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20
Q

Draw the mechanism for nucleophilic substitution with a hydroxide ion.

A
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21
Q

Draw the mechanism for nucleophilic substitution with a cyanide ion nucleophile

A
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22
Q

Draw the mechanism for nucleophilic substitution with an ammonia nucleophile.

A
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23
Q

What a substitution reaction?

A

A reaction when an atom or group replaces another atom or group on the molecule

24
Q

what is an addition reaction?

A

A reaction where a group/atom is added to a unsaturated molecule to produce a saturated molecule.

25
Q

what is an electrophile?

A

A lone elctron pair acceptor

Usually cations or delta positive

26
Q

What happens during an elimination reaction?

A

A hydrogen and halogen are removed from the saturated alkane to form an unstaurated alkene, water and a salt.

27
Q

What is a base?

A

A proton (H+) acceptor

28
Q

what is an acid?

A

A proton (H+) donor

29
Q

What is an alkali?

A

A soluble base.

Produces OH- ions in water.

30
Q

What are the conditions for an elimaination reaction?

A

hot, ethanolic, concentrated potassium hydroxide

31
Q

What type of halogenoalkane favours elimination/nucleophilic substitution?

A

Primary favours nucleophilic substitution

Tertiary favours elimination

32
Q

what does the hydroxide ion act as in a elimination reaction?

A

A base

33
Q

What hapens during electrohilic addition?

A

The carbon doudle bond opens and an atoms/groups are added to each carbon atom. This forms a saturated alkane from and unsaturated alkene.

34
Q

Name the intermediate of electrophilic addition

A

A carbocation

35
Q

What is the major/minor product?

A

Major - the product in highest abundance

minor - the product in lowest abundance

36
Q

Why are alkenes suseptible to electrophilic addition?

A

The carbon-carbon double bond occurs in the overlap of two p orbitals. This creates an elctron rich π bond which attracts the positively charged electrophile.

37
Q

What is the positive inductile effect?

A

The ability of a alkyl group to repel electrons away from itself.

38
Q

How do you determin the major product from electrophilic addition?

A

The more stable carbocation will form the major product.

The more stable carbocation will by have the reaction occuring at a carbon attached to more alkyl groups (increased positive inductile effect). These will push more electrons towards the carbocation

39
Q

Give the conditions for electrophilic addition with bromine

A

aqueous

40
Q

Give the conditions for electrophilic addition with sulfuric acid

A

Concentrated sulfuric acid (H2SO4), cold (room temperature)

41
Q

Give the product for electrophilic addition with sulphuric acid

A

an alkyl hydrogen sulphate

42
Q

What makes the product of electrophilic addition with sulphuric acid useful?

A

The product is an alkyl hydrogen sulphate. Adding water will give an alcohol (and sulphuric acid)

43
Q

Give the conditions for an elctrophilic addition reaction with hydrogen bromide

A

cold (room temperature)

44
Q

Give the reaction conditions for elctrophilic addition with water with a strong acid.

A

strong acid must be dissolved in the water.

45
Q

what is meant by a strong acid?

A

An acid that fully dissociates in water, gives a large amount of H+ ions in solution.

46
Q

what is interesting about electrophilic addition with water and a strong acid.

A

H+ ions from strong acid acid as electrophile

Produces an intermediate carbocation. This reacts with the lone pair of an oxygen on a water molecule.

A second intermediate is produced with a cation oxygen. This attracts the electrons in the oxygen hydrogen bond so hydrogen ions leave.

Hydrogen ions act as a catalyst.

47
Q

draw the reaction mechanism for an elimination reaction

A
48
Q

draw the mechanism for a bromine electrophilic addition reaction

A
49
Q

draw the mechanism for an hydrogen bromide electrophilic addition reaction

A
50
Q

draw the mechanism for elctrophiic addition in water with a strong acid

A
51
Q

draw the reaction mechanism for electrophilic addition with sulphuric acid.

A
52
Q

Draw the structure of sulphuric acid

A
53
Q

Give all the possible electrophiles in electrophilic addition.

A

bromine (istantaneous dipole forms delta positive)

Hydrogen brominde (hydrogen delta positive)

Sulphuric acid (hydrogen delta positive)

Water with strong acid (hydrogen ion)

54
Q

Name the organic product from an electrophilic addition reaction with sulphuric acid

A

alkyl hydrogen sulphate

55
Q

name the organic product from an electrophilic addition with water and a strong acid

A

an alcohol

A-level chemistry my notes (7 decks)

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